1. Field of the Invention
The present invention relates to a control rod position detector disposed in a boiling-water reactor pressure vessel.
2. Background Art
With reference to FIG. 5, a general structure of a boiling water reactor as a light water reactor is described. A plurality of fuel assemblies 2 are loaded in a reactor pressure vessel 1 to form a reactor core 3. Control rods 4 are independently inserted from below into the reactor core 3 and drawn therefrom by a control rod driving mechanism 5, whereby an output of the reactor core 3 is controlled. A plurality of control rod driving mechanism housings 6 are arranged on a lower end plate of the reactor pressure vessel 1. The control rod driving mechanism 5 is inserted from below into the control rod driving mechanism housing 6 to be fixed thereto.
As shown in FIG. 6, a conventional control rod position detector is disposed in an indicator tube 7 in the control rod driving mechanism. Reed switches 8 that are opened and closed by a magnetic force are fixed at regular intervals on an inside of the control rod position detector. A driving piston 9 of the control rod driving mechanism includes a ring magnet 10. When the ring magnet 10 is moved in accordance with a vertical movement of the driving piston 9, the reed switch 8 resting on a position corresponding to a position of the ring magnet 10 is excited by the ring magnet 10 and it is then made conductive. The position of the control rod is detected by the conduction signal.
However, during a manufacturing process, a foreign substance may enter an inside of the reed switch, which is used in the control rod position detector. The foreign substance located between contacts of the reed switch interrupts conduction. The reed switch is formed by enclosing two reeds made of a magnetic material in a glass tube, with a predetermined amount of space between the two reeds. Thus, when an impact is given to the control rod position detector while handling the same, a distance between the reeds may change, which leads to a change in sensitivity. Besides, when the glass tube is cracked, an inert gas contained therein may leak to oxidize the contact portions of the reed. This may give rise to an increased contact resistance to interrupt conduction. In these cases, a position of the control rod cannot be detected.
The present invention has been made in view of these disadvantages. The object of the present invention is to provide a control rod position detector that is capable of avoiding and preventing a situation which makes impossible a detection of a position of the control rod, the situation being caused by the above-described reasons.